1. Field of the Invention
The present invention relates to a non-reciprocal circuit element and a communication device, and in particular, to a non-reciprocal circuit element with a reduced shift of a center frequency of an insertion loss with change in temperature.
2. Description of the Related Art
Lumped constant isolators are high-frequency components allowing signals to pass in the transmission direction without loss and preventing the signals from passing in the opposite direction. The isolators are disposed between a transmitting circuit and an aerial for use in a communication device, such as a cellular phone.
The isolator mainly includes a magnetic plate, three main segments folded around the magnetic plate, and a magnet for applying a bias magnetic field to the magnetic plate. The magnetic plate is composed of, for example, yttrium-iron-garnet ferrite (hereinafter referred to as YIG ferrite (basic composition Y3Fe5O12)), and the magnet is composed of ferrite.
A related technical document of the isolator includes, for example, Japanese Unexamined Patent Application Publication No. 11-283821.
A temperature coefficient of the saturation magnetization of a typical YIG ferrite is about −0.27%/° C. in a temperature range from −35° C. to 85° C. A temperature coefficient of the residual magnetization of the ferrite magnet is about −0.18%/° C. in the same temperature range. The absolute value of the difference between both of the temperature coefficients is about 0.09. That is, the decreasing rate of the saturation magnetization of the YIG ferrite is widely larger than the decreasing rate of the residual magnetization of the magnet. Therefore, the ratio of the residual magnetization of the magnet to the saturation magnetization of the YIG ferrite becomes large as temperature decreases. Unfortunately, this phenomenon decreases the inductance of the main segments, widely shifts a center frequency of the insertion loss from the preset value, and increases the insertion loss of the isolator.